Reuse `User#notification_settings_for` when it's possible

app/controllers/groups/notification_settings_controller.rb

 class Groups::NotificationSettingsController < Groups::ApplicationController
   def update
-    notification_setting = group.notification_settings.find_by(user_id: current_user)
+    notification_setting = current_user.notification_settings_for(group)
     saved = notification_setting.update_attributes(notification_setting_params)
 
     render json: { saved: saved }

app/controllers/projects/notification_settings_controller.rb

 class Projects::NotificationSettingsController < Projects::ApplicationController
   def create
-    notification_setting = project.notification_settings.new(notification_setting_params)
-    notification_setting.user = current_user
-    saved = notification_setting.save
+    notification_setting = current_user.notification_settings_for(project)
+    saved = notification_setting.update_attributes(notification_setting_params)
 
     render json: { saved: saved }
   end
 
   def update
-    notification_setting = project.notification_settings.find_by(user_id: current_user)
+    notification_setting = current_user.notification_settings_for(project)
     saved = notification_setting.update_attributes(notification_setting_params)
 
     render json: { saved: saved }

app/models/member.rb

@@ -167,7 +167,7 @@ class Member < ActiveRecord::Base
   end
 
   def notification_setting
-    @notification_setting ||= user.notification_settings.find_by(source: source)
+    @notification_setting ||= user.notification_settings_for(source)
   end
 
   private

app/services/notification_service.rb

@@ -355,10 +355,10 @@ class NotificationService
     users.reject do |user|
       next user.notification_level == level unless project
 
-      setting = user.notification_settings.find_by(source: project)
+      setting = user.notification_settings_for(project)
 
       if !setting && project.group
-        setting = user.notification_settings.find_by(source: project.group)
+        setting = user.notification_settings_for(project.group)
       end
 
       # reject users who globally set mention notification and has no setting per project/group

spec/services/notification_service_spec.rb

@@ -89,8 +89,8 @@ describe NotificationService, services: true do
           note.project.group.add_user(@u_watcher, GroupMember::MASTER)
           note.project.save
 
-          @u_watcher.notification_settings.find_by(source: note.project).participating!
-          @u_watcher.notification_settings.find_by(source: note.project.group).global!
+          @u_watcher.notification_settings_for(note.project).participating!
+          @u_watcher.notification_settings_for(note.project.group).global!
           ActionMailer::Base.deliveries.clear
         end